Ligand-Promoted Photoreductive Dissolution of Goethite by Atmospheric Low-Molecular Dicarboxylates

Recent evidence suggested that organic ligands in atmosphere water play an important role in the mobilization of iron from mineral aerosol. In this study, the dissolution of goethite (α-FeOOH) was investigated in the presence of three low-molecular dicarboxylates enriched in the atmosphere, as well as a reference organic acid of methanesulfonate (MSA), which is especially abundant in marine atmospheric boundary layer. Iron mobilized from α-FeOOH was promoted under the irradiation and the deaerated condition, and the soluble Fe(II) concentration was enhanced greatly in the ligand-containing suspensions exposed to light. Irrespective of the reaction conditions, the capacities of the dicarboxylates on Fe mobilization were in the following order: oxalate > malonate > succinate, which were closely correlated with the carbon chain length of dicarboxylates: n = 2 > 3 > 4. The space barrier action of carbon atoms inhibited ligand-promoted Fe dissolution by affecting the structure and stability of the complexes. MSA also acted as an organic ligand to mobilize iron and showed weak capacity to reduce Fe(III) under the irradiation. The reactive oxygen species (ROS) analysis indicated that ·OH, O2·–, and H2O2 could be involved in the Fe(II)–Fe(III) redox circle, and the ligand-promoted photoreductive dissolution process could be an important source of ROS in atmosphere water. Both transmission electron microscopy analysis and zeta potential data supported that the adsorption of oxalate molecules onto the surface would change the aggregation state of goethite nanoparticles, which increased the effective surface area, and therefore facilitated Fe mobilization from the oxide. The data shown herein deepens our understanding on the ligand-promoted dissolution mechanisms, which could be an important formation pathway of bioavailable Fe in the atmosphere.